I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to an apparatus and method for supporting analog fax calls between two wireless subscriber stations in a tandem configuration.
II. Description of the Related Art
In a fax communication system, two fax machines generally communicate with each other through a standard public switched telephone network (PSTN) channel. A digital communication system or satellite link may be incorporated within the fax system to extend the range of coverage, allow mobility for the communicating devices, and increase interconnectivity between different communicating devices. An exemplary digital communication system which can be used in conjunction with a fax communication system is a terrestrial wireless system or a satellite communication system.
In wireless mobile and wireless local loop networks, a tandem configuration is established when calls from one wireless subscriber to another wireless subscriber are routed through the PSTN. A fax machine may be connected to each of the wireless subscribers. In this configuration, the base station is generally unaware that it is communicating with another base station across the PSTN.
The tandem configuration may be necessary for several reasons. First the base station communicating with one subscriber may not be aware that the other subscriber is also a wireless subscriber. Thus, the second base station is necessary for providing a link to the other subscriber. Also, two base stations may be necessary because the two wireless subscribers may not be located in the same coverage area of any one base station. Further, for billing purposes, the call should pass through the PSTN, which contains the billing center.
Although necessary for connecting two wireless subscribers, tandem configurations cause degradation in performance. For voice calls, voice quality is degraded due to the multiple encoding and decoding which occurs as the speech signal is passed from the subscriber unit to the base station to the receiving base station and the receiving subscriber unit. For fax calls, the processing between the two base stations degrades performance by adding errors and delays.
When two fax machines communicate directly through a standard PSTN channel, channel quality and delay characteristics are well known and manageable. However, in a tandem configuration, processing delays result from the large amount of digital signal processing and from transmission delays. Further delays are incurred as the base stations combine or multiplex signals from many communicating devices into a common transmission signal. The resultant overall delay can be both intolerably long and unpredictable.
The source of the delay and the problems associated with the delay will be better appreciated through an understanding of the processing associated with a fax call. For a standard Group 3 fax communication system, the timing and synchronization between fax machines conform to behavior as specified in "ITU-T Recommendation T.30: Procedures for Document Facsimile Transmission in the General Switched Telephone Network," hereinafter referred to as the T.30 fax protocol, and incorporated by reference herein. The T.30 fax protocol uses a number of modulation techniques for transmission of forward message data. In particular, parameter negotiation and handshaking between fax machines is accomplished using the modulation technique specified in "CCITT Recommendation V.21: 300 bps Duplex Modem Standard for use in the General Switched Telephone Network (GSTN)" hereinafter referred to as V.21, and also incorporated by reference herein. The handshaking establishes the proper mode of communications between the fax machines.
For a T.30 fax system, V.21 messages are sent between a calling fax machine and a called fax machine in establishing a call. A calling fax machine initiates a call by dialing the called fax machine and sending a calling tone (CNG). The called fax machine detects the incoming call and sends a called station identification tone (CED) back to the calling fax machine. The called fax machine then sends its digital identification signal (DIS) to the calling fax machine to inform the calling fax machine of its capabilities. Upon detection of the DIS signal, the calling fax machine sends a digital command signal (DCS) to inform the called fax machine of the capabilities the calling fax machine plans to use.
Other messages in addition to the above described initiation signals also take place between the fax machines during a call. For example, training signals, information messages, and termination messages are also part of a typical fax call.
According to T.30, whenever a fax sends a V.21 message, it must first send a preamble having a one second duration. The preamble consists of a series of V.21 flags. The preamble is used to inform the receiver that a message is to be sent, and allows the receiver to synchronize correctly so that the actual message may be properly received.
Although necessary for synchronization, the preamble delays the message for at least one second. A tandem configuration further contributes to the delay. Both the preamble and the message must be transmitted over the PSTN link, and the additional processing delay associated with the transmission may result in an unacceptably long delay. Furthermore, the additional delay may result in a prematurely dropped call.
The delay due to the tandem configuration may also cause a message collision to occur. A message collision occurs when a message and a response are being transmitted at the same time. The T.30 fax protocol specifies the sequence and format of the messages, such as the CNG, CED, DIS, and DCS messages, that are transmitted. To accommodate unfavorable channel conditions, the T.30 protocol requires that certain unanswered messages from a source fax machine be repeated. A source fax machine may refer to either the calling or the called fax machine. After a source fax machine sends a message to the destination fax machine, the source fax machine expects a response from the destination fax machine within a specified period of time. If no response is received after the specified period of time, the source fax machine retransmits the message. The retransmissions continue until a response from the destination fax machine is received or an excessive number of attempts have been made.
According to the T.30 fax protocol, the repetition interval for a fax machine operating in automatic mode is 3.0 sec.+-.0.45 sec. This means that a fax machine conforming to the T.30 fax protocol should not retransmit a message within 2.55 sec of the prior message. Thus, no collision will occur if such a fax machine receives a response within the shortest repetition interval of 2.55 sec.
However, long transmission delays in the communication channel can cause message collisions. In one scenario, the source fax machine sends a message to the receiving fax machine, and because of transmission delays, the destination fax machine may not respond right away. The transmission delay may be due to the extra time needed for the base station to communicate with the fax machine over-the-air. Since the source fax machine does not receive the response in time, it retransmits the message. The source fax machine will retransmit the message approximately every three seconds. In the meantime, the destination fax machine may respond. If the response from the destination fax machine arrives at the same time the source fax machine is retransmitting the message, a message collision results and the response will not be received by the source fax machine. The message collision may ultimately result in the fax machines dropping the call.
The lengthy delays and the potential for message collisions in a tandem configuration need to be addressed for fax machines to function reliably in a network involving more than just the PSTN. These problems and deficiencies are solved by the present invention in the manner described below.